Our broad goals are to understand the interactions of Epstein-Barr virus (EBV) with its host cell. EBV maintains a persistent but intermittently reactivating infection in B lymphocytes and contributes to the development of lymphoid and epithelial malignancies affecting millions of humans worldwide. How EBV gene expression modulates cellular gene expression. to maintain a homeostatic balance with the host immune response is the overall focus of our proposal. A key regulator of host cell and EBV gene expression is the early lytic EBV protein SM. SM is a nuclear protein essential for EBV lytic replication that enhances mRNA stabilization, export and processing. EBV SM also regulates cell and EBV gene expression by directing alternative splicing in a highly specific manner. We have shown that SM acts to alter splicing of cellular STAT1 mRNA and change the pattern of STAT1 expression. Our observations therefore indicate that SM may modulate host innate immune responses during the reactivation and lytic replication of EBV. Understanding the key players in viral effects on alternative splicing has the potential to open up new targets for inhibiting virus replication using directed small molecules or RNAs. We have recently found that certain interferon-stimulated genes (ISGs) induced by SM inhibit EBV replication by targeting specific EBV genes. These cytoplasmic ISGs are induced during EBV reactivation, suggesting the novel hypothesis that they may detect and act on reactivating latent viruses when they produce viral nucleic acids and proteins that exit the nucleus. A major hypothesis of this application is that cells employ innate immunity molecules to combat reactivation of internal latent viruses in ways different from those used for external pathogens. This is a new paradigm for the interaction of the innate immune system with latent infections that may broadly apply to herpesviruses and other persistent pathogens. This proposal therefore has the potential to reveal basic mechanisms of action of novel effectors with antiviral activity. Understanding new points of intersection between the immune response and EBV reactivation is important for devising new treatment strategies. We therefore propose studies with the following two specific aims: 1. Determine the mechanisms by which interferon-stimulated genes (ISGs) inhibit or enhance EBV replication. 2. Define the role of alternative splicing and cellular RNA binding proteins in EBV lytic replication and gene transactivation by SM.